1. Field of the Invention
The present invention relates to an improvement of a mode S secondary surveillance radar system capable of detecting aircraft mounted with a mode S transponder or an ATCRBS transponder.
2. Description of the Related Art
Targets (aircrafts) of a secondary surveillance radar system (referred hereafter as SSR) of a ground station include an aircraft mounted with an ATCRBS transponder and an aircraft mounted with a mode S transponder.
The SSR for detecting the ATCRBS transponder and the mode S transponder carries out the surveillance in which a beam dwell time T is divided into a plurality of sweeps (four sweeps, for example), and each sweep is divided into an all-call period and a roll-call period.
For example, as described in Takashi Yoshida (ed.): “Revised Radar Technology”, the Institute of Electronic, Information and Communication Engineers, published Oct. 1, 1996, in the all-call period, a mode A/C specialized transaction detecting an aircraft mounted with the ATCRBS transponder and a mode S specialized collective transaction detecting an aircraft mounted with the mode S transponder will be carried out. In the roll-call period, the mode S individual interrogation (roll call) and reply will be carried out by such a schedule that the transaction for respective targets will not overlap, according to a position information (range, azimuth) and a mode S address of targets (aircrafts) obtained from replies of the transponders to the mode S specialized collective interrogation.
Note that the beam dwell time T is a time corresponding to an angle range based on a rotational speed of an antenna that is scanning in a direction of the SSR (azimuth) and a transmission beam width, which is defined uniquely.
FIG. 1 is a schematic diagram showing a configuration of the conventional mode S secondary surveillance radar system and a positional relationship with a target (aircraft) to be detected by the mode S secondary surveillance radar system, and FIG. 2 is a diagram showing a concrete configuration of a processor 3 shown in FIG. 1.
As shown in FIG. 1 and FIG. 2, the mode S secondary surveillance radar system (ground station) on the ground which is a sensor comprises an antenna 1, a transceiver 2 connected to the antenna 1 and formed by a transmission and reception switch 21, a transmitter 22 and a receiver 23, and a processor 3 with a built-in computer connected to the transceiver 2, where the transceiver 2 and the processor 3 constitute an interrogator.
The processor 3 comprises a transmission controller 31 connected to the transmitter 22, a mode S reply processor 32 and an ATCRBS reply processor 33 connected to the receiver 23, a channel manager 34 connected to the mode S reply processor 32 and the transmission controller 31, a surveillance processor 35 connected to the channel manager 34, the transmission controller 31, the mode S reply processor 32 and the ATCRBS reply processor 33, and a timing signal generator 36 for generating and supplying timing signals for scan operations in a direction of the antenna 1 (azimuth), the interrogation transmission, etc., in order to realize an overall control of a system of the processor 3 as a whole.
On the other hand, the transponder of the target (aircraft) comprises an antenna 4 for receiving a interrogation transmitted from the antenna 1 of the ground station (mode S secondary surveillance radar system) and transmitting a reply to that interrogation, i.e. a mode S reply or a mode A/C reply, towards the ground station, a transceiver 5 connected to the antenna 4, and a signal processor 6 connected to the transceiver 5, where the transceiver 5 and the signal processor 6 constitute a transponder.
Note that, an own site ID (identification code) that is necessary in decoding the reply from the target is set in advance in the mode S reply processor 32 of the processor 3, and parameters such as PR (reply probability), a target information obtained by a plurality of scans when a so called target information such as a range (distance), an azimuth and an altitude regarding the target position is obtained over a plurality of scans, and a parameter to become a reference for judging whether it is within a correlated range among scans, i.e. a correlation value are set in advance and stored in the surveillance processor 35.
The channel manager 34 allocates the all-call period TA and the roll-call period TR in the beam dwell time T according to signals from the timing signal generator 36.
The transmission controller 31 generates the mode S specialized collective interrogation and the mode A/C specialized interrogation of the reply probability 1 in the all-call period according to the own site ID and the PR value supplied from the surveillance processor 35, and transmits them towards the target (aircraft) from the antenna 1 through the transmitter 22 and the transmission and reception switch 21.
Then the collective interrogation (of the mode S specialized one plus the mode A/C specialized one) from the ground station is transmitted towards the target mounted with the mode S transponder, and the reply to that transmitted collective interrogation from the transponder is received through the antenna 1 and the transmission and reception switch 21 of the ground station, digitized by the receiver 23 which has amplification, detection and quantization functions, and supplied to the mode S reply processor 32 and the ATCRBS reply processor 33.
The mode S reply processor 32 carries out the preamble detection and the message decoding for the received reply X, estimates a distance to the target and a direction of the target from the position information of the target that can be obtained from the reception timing of the reply X, judges whether the own site ID and the PI (Parity/Interrogator ID) field coincide or not according to a mode S address contained in the reply X, and supplies the reply X for which the own site ID and the PI field coincide to the channel manager 34 and the surveillance processor 35 as a processing target reply. Here, data for which the own site ID and the PI field do not coincide, i.e. non-coincident reply data, will be discarded as not a processing target.
Then, at the surveillance processor 35 which received a supply of the reply of the target which is judged as the processing target at the mode S reply processor 32, a detection report regarding that target is produced, registered and outputted, for those replies for which the mode S all-call coincides among the replies obtained in the same direction and the distance and the direction are within predicted ranges in the previous scan of the antenna 1 and which are within the correlated range that is set in advance. Here, the reply data which are not correlated will be discarded.
Note that, at the surveillance processor 35, if there exists targets for which the mode S address overlaps among targets within the control coverage area controlled by that ground station, in relation to the replies supplied from the mode S reply processor 32, then a case where the air traffic control cannot be carried out properly as one of them is false may occur, so that the acquiring by the all call is carried out again for all the overlapping targets in the case where the mode S address overlaps.
Then, the surveillance processor 35 outputs a command to the channel manager 34, such that the individual interrogation (roll-call) containing a command of a lockout for not responding to the all call will be scheduled, in relation to the target of the reply which is judged as correlated.
The channel manager 34 that received the command carries out the scheduling such that the transaction will not overlap, sequentially from the target (aircraft) for which the distance is farther, for the purpose of the effective utilization of the RF channels.
In the roll-call period, the selective interrogation is scheduled and its reply is received in order to acquire the mode A code information and the altitude information along with the mode S address, for the target acquired by the all call.
For the individual reply of the mode A code information and the altitude information received, the mode S reply processor 32 carries out the preamble detection and the message decoding, sets the mode S address acquired by the earlier all call as a predicted mode S address, and judges whether the predicted mode S address and the mode S address acquired by the roll call coincide or not.
Here, the mode S reply processor 32 supplies the reply for which the mode S address coincides as a processing target to the surveillance processor 35, so that the surveillance processor 35 initially acquisition a target (aircraft) through the correlation processing, and then carries out the production, registration and output of the target detection report according to the acquired mode A code information at the roll-call, and stores code data of that acquired mode A code information into a memory such as RAM. Note here that data of the reply which is not correlated will be discarded as not a processing target.
Note that the mode A code information stored in a memory at the surveillance processor 35 is unique to a flight of the target (aircraft), so that usually it will not be changed while the surveillance and the control of that target are to be continued.
For this reason, in the continued surveillance of the target by the scanning of the antenna 1 after the mode A code information is acquired by the selective interrogation soon after the initial acquiring, the transaction for acquiring the mode A code information will be omitted in order to increase the utilization efficiency of the RF channels.
Consequently, in the production of the target detection report at the scan after the initial acquiring, the detection report for that target is produced by reading out the mode A code information acquired soon after the initial acquiring and stored in a memory whenever necessary.
However, the mode A code information of the target will be acquired again, stored in a memory and utilized by reading it out, when acquired again soon after the coast occurs or when the flight status (FS) of the target changes, for example.
As described above, the mode A code information unique to the flight of the target (aircraft) is usually changed very rarely, so that in the conventional mode S secondary surveillance radar system, the interrogation for acquiring the mode A code information will be omitted in the selective interrogation at the scan of the antenna 1 after the initial acquisition after acquired the mode A code information.
Also as described above, in the conventional mode S secondary surveillance radar system, the mode A code information acquired first that target is used repeatedly in the target detection report production by the scan of the antenna 1 after the initial acquisition, so that if the code data of the mode A code information acquired first contain erroneously converted characters or errors due to some other causes, this erroneous mode A code information would be adopted again, such that there arises a possibility that the air traffic control will not be carried out properly.